Selection of materials for certain applications, such as packaging or electronics for example, requires study of the permeation of certain gases through these materials. By permeation, is meant the mechanism for letting a gas through a material according to gas absorption steps in the material, by diffusion steps of this gas through the material, and by desorption of the gas on the other side of the material. For example, gas permeation measurements for oxygen or steam (i.e. water vapor), through materials to be selected, are the most widespread.
In the case of materials intended for agro-food packaging for example, the study of the permeation of common gases through the materials, and more particularly oxygen and steam, are critical. The required levels of permeability to these gases are extremely low, and the study of permeation therefore requires devices for measuring permeation having significant sensitivities.
In response to this problem, many devices for measuring permeation have been developed, based on various principles for tracking gases, and each having their drawbacks.
In particular a device for measuring permeation flows has been developed, comprising a measurement enclosure in a high vacuum, the bather film for which permeation is to be studied, is placed at the interface between two chambers. The upstream chamber to the barrier film is filled with a controlled pressure of the targeted gas, for example steam or oxygen. A vacuum is applied to the downstream chamber and coupled with a measurement apparatus capable of detecting the target gas having diffused by permeation through the sample to be tested, such as for example a mass spectrometer. The gas present in the upstream chamber is transmitted into the downstream chamber by a permeation process and measurement of permeation of the film consists in detecting the transmitted flow by the detection means set into place on the downstream side. The detection of the flow consists in measuring an ionization current corresponding to the mass of the target gas. The measurement is then converted into a gas flow transmitted per unit surface J(t) (in g·m−2·d−1) by using a reference sample or a reference calibrated leak which generates a controlled flow of target gas.
In order to increase the sensitivity of the measurements conducted with such devices, it was proposed to use, for a gas for which one tries to determine the permeation through the material, an isotope gas of this gas, i.e. having a different mass number. Thus, by for example using a mass spectrometer as an analyzer in the measurement enclosure, it is possible to reduce the detection thresholds of the permeation by several orders of magnitude. Indeed, the natural isotope abundance of these elements being very low, the contamination of the enclosure with these species is all the less significant.
U.S. Pat. No. 6,624,621 describes a device for measuring permeation taking up again the principles mentioned above and which further has the advantage of carrying out permeation measurements for several different gases simultaneously, which gives the possibility of having measurements with good sensitivity and makes the selection of the materials more rapid.
The use of a mass spectrometer as a measurement means is actually well adapted since it allows measurement of a large variety of gases and the use of an isotope as a target gas gives the possibility of accessing great adequate measurement sensitivity for measuring highly barrier-forming materials. A mass spectrometer is an analysis means allowing detection and identification of molecules of interest by measuring their mass. In an enclosure placed in a dynamic vacuum situation, the mass spectrometer allows the measurement of the pressures of the residual gases. In the case of a permeation measurement, the permeation flow of the sample will be expressed by an increase in the residual pressure of the target gas up to a stabilized value which is the permeation value. The signal from the mass spectrometer corresponds to the ionization current (optionally amplified) of the masses of interest corresponding to the target gas for which one tries to determine the permeation through the sample. For example, the mass 4 will be measured for helium, the mass 18 for water or further the mass 20 for heavy water.
However, it was ascertained that the ionization current of the mass spectrometer for a given mass may vary over time without this corresponding to an actual variation of the partial pressure in the enclosure. This observation was established by conducting permeation measurements to helium gas with a reference material (for example a polyester film stored under controlled conditions). The measurement then corresponds to the ionization current of the mass 4 in the stabilized condition of permeation. FIG. 1 shows a set of such measurements conducted under strictly identical conditions (temperature, upstream helium pressure, reference sample). In FIG. 1, is illustrated the control card for the measurement of permeation to helium of the reference material. A significant variability of the ionization current among the different measurements is seen while the latter should be identical.
This variability forces a systematic control on the reference before any series of measurements in order to calibrate the apparatus and to be able to compare the measurements with each other. These steps for calibrations are made before and/or after the measurement of permeation on the sample to be tested. For example it is possible to use a reference film or to use calibrated leaks which deliver a controlled amount of target gas in the measurement enclosure and thus allow calibration of the mass spectrometer. Patent application US 2010/0223979 gives an example of a method wherein calibration is carried out before permeation measurements as such, this calibration may for example be conducted with a flow of a mixture of gases in which the target gas is highly diluted, i.e. present in a very small amount. These calibration steps are however a problem since they will significantly extend the durations of the permeation measurements which already are very long. Further, they are not totally satisfactory since they do not allow fine correction of the different modifications of the signal from the mass spectrometer which may occur during the permeation measurement.
An object of the present invention is therefore to propose an improved method for measuring permeation and an associated device which give the possibility of solving at least one of the aforementioned drawbacks.
More particularly, an object of the present invention is to propose an improved method for measuring permeation and an associated device, which give the possibility of providing reliable and accurate permeation measurements, while not extending the usual measurement times.